Wireless precoding methods

ABSTRACT

Various wireless precoding systems and methods are presented. In some embodiments, a wireless transmitter comprises an antenna precoding block, a transform block, and multiple transmit antennas. The antenna precoding block receives frequency coefficients from multiple data streams and distributes the frequency coefficients across multiple transmit signals in accordance with frequency-dependent matrices. The transform block transforms the preceded frequency coefficients into multiple time domain transmit signals to be transmitted by the multiple antennas. The frequency coefficients from multiple data streams may be partitioned into tone groups, and all the frequency coefficients from a given tone group may be redistributed in accordance with a single matrix for that tone group. In some implementations, the frequency coefficients within a tone group for a given data stream may also be precoded. In some alternative embodiments, tone group preceding may be employed in a single channel system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 13/237,543filed Sep. 20, 2011, now U.S. Pat. No. 8,693,575, which is aContinuation of application Ser. No. 11/199,629 filed Aug. 9, 2005, nowU.S. Pat. No. 8,023,589, which is a non-provisional application claimingpriority to U.S. Provisional Application Ser. No. 60/599,935 filed onAug. 9, 2004, entitled “Closed Loop Techniques for MIMO AndConstellation Pre-Rotation For OFDM,” which is hereby incorporated byreference. This application also relates to U.S. patent application Ser.No. 11/182,083, filed on Jul. 1, 2005, entitled “Method And ApparatusFor Providing Closed-Loop Transmit Precoding,” which is herebyincorporated by reference herein.

BACKGROUND

Multiple Input, Multiple Output (MIMO) refers to the use of multipletransmitters and receivers (multiple antennas) on wireless devices forimproved performance. When two transmitters and two or more receiversare used, two simultaneous data streams can be sent, thus doubling thedata rate. Various wireless standards that are based on MIMO orthogonalfrequency-division multiplexing (OFDM) technology use an open loop modeof operation. In the open-loop MIMO mode of operation, the transmitterassumes no knowledge of the communication channel. Although theopen-loop MIMO mode may be simpler to implement, it suffers performanceissues. An alternative to open-loop mode is closed-loop processing,whereby channel-state information is referred from the receiver to thetransmitter to precode the transmitted data for better reception.Closed-loop operation offers improved performance over open-loopoperation, though not free of cost. The transmission of channel-stateinformation from the receiver to the transmitter involves significantoverhead. It is desirable, therefore, to design a reduced-feedbackclosed-loop mode of operation with the performance similar to thatobtained using the full channel-state information feedback.

SUMMARY

Accordingly, there is disclosed herein various wireless precodingsystems and methods. In some embodiments, a wireless transmittercomprises an antenna precoding block, a transform block, and multipletransmit antennas. The antenna precoding block receives frequencycoefficients from multiple data streams and distributes the frequencycoefficients across multiple transmit signals in accordance withfrequency-dependent matrices. The transform block transforms theprecoded frequency coefficients into multiple time domain transmitsignals to be transmitted by the multiple antennas. The frequencycoefficients from multiple data streams may be partitioned into tonegroups, and all the frequency coefficients from a given tone group maybe redistributed in accordance with a single matrix for that tone group.In some implementations, the frequency coefficients within a tone groupfor a given data stream may also be precoded. In some alternativeembodiments, tone group precoding may be employed in a single channelsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustrative multiple input, multiple output (MIMO)system;

FIG. 2 is a function block diagram of an illustrative open loop MIMOsystem with antenna precoding;

FIG. 3 a is a function block diagram of an illustrative closed loop MIMOsystem with antenna precoding;

FIG. 3 b is a function block diagram of an illustrative alternativeclosed loop MIMO system with antenna precoding;

FIG. 4 is a function block diagram of an illustrative closed loop MIMOsystem with antenna precoding in the frequency domain;

FIG. 5 is a function block diagram of an illustrative open looporthogonal frequency division multiplexing (OFDM) system with tone groupprecoding;

FIG. 6 is a function block diagram of an illustrative closed loop OFDMsystem with tone group precoding;

FIG. 7 is a function block diagram of an illustrative open loop MIMOOFDM system with antenna and tone group precoding;

FIG. 8 is a function block diagram of an illustrative MIMO OFDM systemwith closed loop antenna precoding and open loop tone group precoding;

FIG. 9 is a function block diagram of an illustrative MIMO OFDM systemwith closed loop antenna precoding and closed loop tone group precoding;

FIG. 10 is a function block diagram of an illustrative open loop MIMOOFDM system with combined antenna and tone group precoding; and

FIG. 11 is a function block diagram of an illustrative closed loop MIMOOFDM system with combined antenna and tone group precoding.

DETAILED DESCRIPTION

FIG. 1 shows a portable digital device 102 that communicates wirelesslywith a second digital device 104. Portable digital device 102 can be anyone or more of the following example devices: a tablet computer, alaptop computer, a personal digital assistant (“PDA”), a mobile phone, aportable digital music player, a digital camera, and a remote control.Second digital device 104 can be any one or more of the followingexample devices: a wireless network base station, a host computer; amobile phone base station, a home appliance, and a second portabledigital device.

Portable digital device 102 includes two or more antennas 106, 108. (InFIG. 1, antennas 106, 108 are embedded in the chassis of device 102.)Similarly, the second digital device 104 includes two or more antennas110, 112. Each antenna 106-112 can transmit and receive to exchangewireless signals with every other antenna, though one or more of thechannels may be more highly attenuated than other channels. At the highfrequencies of interest, frequency-selective fading that varies fromchannel to channel may be expected as a matter of course.

A multiple input, multiple output (MIMO) system such as that shown inFIG. 1 can be modeled mathematically using vectors to represent thetransmit signal set and the receive signal set. If the components ofvector s=[s₁, s₂, . . . , s_(P)]^(T) represent the current or voltagetransmit signal values applied to the transmit antennas at a giveninstant, and the components of vector r=[r₁, r₂, . . . , r_(Q)]^(T)represent the current or voltage receive signal values responsivelyproduced in the receive antennas, the MIMO signal model can be expressedas

${r = {{{\sum\limits_{p = 1}^{P}{h_{p}s_{p}}} + w} = {{Hs} + w}}},$where h_(i)=[h_(1i), h_(2i), . . . , h_(Qi)]^(T) is a Q-dimensionalvector containing channel coefficients from ith transmitter to Qreceivers, H=[h₁, h₂, . . . , h_(P)] is the Q×P channel matrix, andw=[w₁, w₂, . . . , w_(Q)]^(T) is the Q-dimensional vector of zero-meannoise with variance σ². The received signal can be processed by usingeither an optimal maximum-likelihood method or a sub-optimal method,such as zero-forcing or linear minimum mean squared error (MMSE)processing. See D. P. Palomar, M. A. Lagunas, and J. M. Cioffi, “OptimumLinear Joint Transmit-Receive Processing for MIMO Channels with QoSConstraints”, which is hereby incorporated by reference, for examples ofsuitable receiver design techniques.

FIG. 2 shows a precoding technique (also termed “rotation” or“pre-rotation”) in which input signals are distributed over multipletransmit antennas. Precoding may be used to compensate for (or even toexploit) interference between the multiple transmit signals. In block202, R input signals are distributed across P transmit signals. Eachdigital-to-analog converter (DAC) 204, 206, 208, converts a respectivetransmit signal to analog form and applies the transmit signal to arespective transmit antenna 210, 212, 214. The precoding operation canbe represented as the following matrix multiplications=Vd,where d=[d₁, d₂, . . . , d_(R)]^(T) is the R-dimensional vector of inputsignals to be transmitted, and V is the P×R antenna precoding matrix.

The antenna precoding matrix V may be tailored to maximize the channelcapacity, but of course this requires some knowledge or assumptionsabout the channel. For the open loop case, V may be simply a P×Pidentity matrix. Nevertheless, this model enables consideration ofclosed-loop and open-loop options within the same framework, and furtherenables consideration of situations in which the number of transmitantennas is greater than or equal to the number of input signals. Withprecoding, the effective (rotated) channel matrix is denoted byH ^(r)=HV.

FIG. 3 a shows antenna precoding in a closed loop MIMO system. The Ptransmit antennas 210-214 send signals that are detected by Q receiveantennas 220-224. Each receive antenna 220-224 produces a receive signalthat is digitized by a corresponding analog-to-digital converter (ADC)226-230. The ADCs 226-230 may each include amplifiers and signal filtersto ensure an adequate signal-to-noise ratio of the digitized signals. Inblock 232, the energy of the receive signals is redistributed to form Routput signals that correspond to the R input signals. A distributionmatrix U inverts the operations of the effective channel matrix H^(r) toestimate the input signals:{circumflex over (d)}=Ur=U(HVd+w),

If perfect channel state information is available at the transmitter,then the transmitted symbols can be precoded with the eigenvectors V ofthe matrix H^(H)H where (•)^(H) denotes conjugate transposition.Alternative precoding and redistribution matrices V and U can beselected as provided in the Palomar reference cited previously, or asprovided in U.S. patent application Ser. No. 11/182,083, filed on Jul.1, 2005, entitled “Method And Apparatus For Providing Closed-LoopTransmit Precoding”. Block 234 performs precoding and redistributionmatrix selection based on channel estimation. The channel estimation mayinclude estimating the input signals (from the output signals) anddetermining the relationship between the received signals and theestimated input signals. The channel estimation may be performed oncewhen a communication link is established, or may be redeterminedperiodically, or may be continuously tracked. The matrix selections maysimilarly be made once, periodically, or continuously updated.

The precoding matrix selection is communicated from the receiver to thetransmitter by a return channel as indicated by the dashed line. Theprecise nature of the return channel from the receiver to thetransmitter is unimportant, and many suitable alternatives exist. Forexample, a separate communications channel may exist between thereceiver and transmitter, or the transmitter and receiver may conducthalf-duplex or full-duplex communications across the wireless MIMOchannel currently under discussion. Nevertheless, to minimize thebandwidth requirements for the return channel, it is desirable tominimize the number of bits used to communicate the selected precodingmatrix to the transmitter.

FIG. 3 b shows an alternative closed loop system configuration. In block250, the transmitter performs precoding and redistribution matrixselection based on an estimation of the channel transfer function. Thechannel estimation may be determined by the receiver (e.g., in block252) and communicated to the transmitter by return channel.Alternatively, the transmitter may determine the channel estimation,e.g., based on duplex communications from the receiver. In thisconfiguration, the selected redistribution matrix is communicated to thereceiver, potentially via the wireless MIMO channel. Alternatively, aseparate channel may be used to communicate the matrix selection to thereceiver. The separate channel can take the form of a narrowband signalbroadcast from a single antenna at a frequency that is dedicated tocarrying configuration information to the receiver.

FIG. 4 shows a broadband MIMO system that employs antenna precoding. Theillustrative system of FIG. 4 employs orthogonal frequency divisionmultiplexing (OFDM) to distribute data bits among evenly-spaced carrierfrequencies, or “tones”. For each of the R data streams, a correspondingbit allocation block 260, 262, 264 apportions bits among the availabletones. As in other OFDM systems, the allocation of bits may be dynamicbased on the measured signal-to-noise ratio for each tone. The bitallocation blocks 260-264 each provide a quadrature amplitude modulation(QAM) constellation signal point for each tone.

Because of the channel's frequency dependence, a separate antennaprecoding matrix 266 may be provided for each tone. The resultingfrequency coefficients for each tone i aref _(i) =V _(i) d _(i),

where f_(i) is an P-dimensional vector of frequency coefficients on theith tone for each antenna, V_(i) is the P×R antenna precoding matrix forthe ith tone, and d_(i) is the R-dimensional vector of input signalconstellation points for the ith tone. However, the channel's frequencydependence is expected to be small for adjacent tones, and accordingly,some embodiments group adjacent tones together and apply one antennaprecoding matrix 266 to each tone group. For example, an illustrativeembodiment divides 256 available tones into 32 groups of eight toneseach, and accordingly, only 32 antenna precoding matrices are employed.Each antenna precoding matrix is applied to each tone in a given tonegroup to distribute that tone's energy in a nearly-optimal way acrossthe multiple transmit antennas 210-214. The set of antenna precodingmatrices 266 produce a set of frequency coefficients for each transmitsignal. Each set of frequency coefficients is transformed into atime-domain digital signal by inverse fast Fourier Transform (IFFT) andparallel-to-serial conversion (PIS) blocks 268-272. The time-domaindigital signals are converted by DACs 204-208 and transmitted viatransmit antennas 210-214.

The receiver includes serial-to-parallel conversion (SIP) and fastFourier Transform (FFT) blocks 280-284 to convert digital receivesignals from the ADCs 220-224 into frequency domain receive signals. Aset of signal distribution matrices 286 inverts the effects of thechannel and antenna precoding matrices 266, yielding{circumflex over (d)} _(i) =U _(i) g _(i) =U _(i)(H _(i) V _(i) d _(i)+w _(i))

where g_(i) is an Q-dimensional vector of frequency coefficients on theith tone for each receive antenna, U_(i) is the R×Q signal distributionmatrix for the ith tone, H_(i) is the Q×P channel response matrix forthe ith tone, and w_(i) is the Q-dimensional vector of channeldistortion on the ith tone. Decoder blocks 288-292 extract the bitinformation from the output signal frequency coefficients. Channelestimation and matrix selection block 294 estimates the channel transferfunction and selects the precoding and distribution matrix setsaccordingly.

Though FIG. 4 shows one closed loop embodiment, alternative embodimentsexist having an open loop implementation or an alternative closed-loopimplementation like that described in relation to FIG. 3 b. Thisobservation also holds true for the other precoding methods and systemsdisclosed hereafter.

FIG. 5 shows tone precoding in an open loop system. The constellationsignal points from bit allocation block 260 are partitioned into tonegroups 302. The signal energy for each tone is distributed across thetone group by a tone group precoding block 304, 306, or 308. The tonegroup precoding may be performed in accordance with the followingequation:f ^(j) =T ^(j) d ^(j)

where f^(j) is an N-dimensional vector of frequency coefficients in thejth tone group for each antenna, T^(j) is the N×N antenna precodingmatrix for the jth tone group, and d^(j) is an N-dimensional vector ofinput signal constellation points for the jth tone group. The outputs ofthe tone group precoding blocks are taken as a set of frequencycoefficients by IFFT and P/S block 268, which converts them into a timedomain digital signal for DAC 204. DAC 204 then drives an analog signalto transmit antenna 210.

Receive antenna 220 provides its received signal to ADC 226, whichconverts the receive signal to digital form. Block 280 transforms thedigital signal into the frequency domain, and blocks 310-314 operate onthe tone groups to invert the effects of the channel and of precodingblocks 304-308. A decode block 288 extracts the data from the frequencycoefficient values in accordance with the following equation:{circumflex over (d)} ^(j) =s ^(j) g ^(j) =s ^(j)(H ^(j) T ^(j) d ^(j)+w ^(j))

where g^(j) is an N-dimensional vector of frequency coefficients on thejth tone group, S^(j) is the N×N signal distribution matrix for the jthtone group, H^(j) is the N×N channel response matrix for the jth tonegroup, and w^(j) is the N-dimensional vector of channel distortion onthe jth tone group.

FIG. 6 shows tone group precoding in a closed loop system. The closedloop system includes the elements of FIG. 5, and further includes ablock 316 to estimate the channel transfer function and select the tonegroup precoding matrices and the tone group signal distributionmatrices. The channel estimation may be performed once when acommunication link is established, or may be redetermined periodically,or may be continuously tracked. The matrix selections may similarly bemade once, periodically, or continuously updated.

FIG. 7 shows a MIMO system having both tone group precoding and antennaprecoding. A set of tone group precoding matrices is provided in blocks320, 322, and 324, to distribute signal energy across the tone groupsfor each of multiple data streams. The outputs of the tone groupprecoding blocks are then taken as the frequency coefficients formultiple transmit signals, and precoded across multiple antennas by aset of antenna precoding matrices 266. In this manner, the data bits aredistributed across multiple frequencies and multiple antennas (multipletransmission paths) to provide a high degree of resistance to fadingwhile maximizing the communications bandwidth of the system.

The frequency coefficients output by block 266 are transformed to thetime domain by IFFT blocks 268-272, converted to analog signals by DACs204-208, and transmitted by antennas 210-214. On the receive side,antennas 220-224 provide analog receive signals to ADCs 226-230, whichdigitize the receive signals. The digitized receive signals aretransformed by FFT blocks 280-284 to obtain the frequency coefficientsof multiple receive signals. Signal distribution matrices 286 extractmultiple data stream signals from the different receive signals, andtone group distribution matrices 330-334 invert the operation of blocks320-324 to obtain constellation signals. Decode blocks 288-292 thenextract data streams from the constellation signals.

The system shown in FIG. 7 is an open loop system, in which the matriceshave been designed for a known or predefined range of assumed channels.FIG. 8 shows a system having closed loop selection of antenna precodingand redistribution matrices, and open-loop predetermined tone groupprecoding and redistribution matrices. Block 336 performs antennaprecoding and redistribution matrix selection based on channelestimation. The channel estimation may include estimating the inputsignals (from the output data streams) and determining the relationshipbetween the received signals and the estimated input signals. Thechannel estimation may be performed once when a communication link isestablished, or may be re-determined periodically, or may becontinuously tracked. The matrix selections may similarly be made once,periodically, or continuously updated.

FIG. 9 shows a MIMO system having closed loop selection of both antennaand tone group precoding matrices. Block 338 performs channel estimationand selection of both antenna and tone group precoding and distributionmatrices based on the channel estimation.

FIG. 10 shows a MIMO system having combined open loop antenna and tonegroup precoding. A set of combined precoding matrices 340 (one matrixper tone group) takes the frequency coefficients for each tone group andredistributes the signal energy over the tone group and the multipleantennas. On the receiving end, a set of combined distribution matrices342 inverts the combined effect of the channel and matrices 340 toreconstruct the frequency coefficients that are then decoded by decodeblocks 228-292.

FIG. 11 shows a MIMO system having closed loop selection of combinedantenna and tone group precoding matrices. Block 344 performs channelestimation and selection of the combined precoding and combineddistribution matrices based on the channel estimation.

It is desirable to minimize the overhead cost associated withcommunicating selected precoding or distribution matrices between thetransmitter and receiver in closed loop systems. Such minimization canbe accomplished in multiple ways. One way to reduce overhead is toreduce the number of bits used to specify the matrices. Each matrixelement may be more coarsely quantized to reduce the number of bits. Inthose embodiments having tone group precoding, the size of the tonegroup precoding and distribution matrices (and hence the overall numberof matrix elements) may be reduced by reducing the number of tones ineach tone group. The tone group precoding and distribution matrices canbe entirely eliminated by employing combined tone group and antennaprecoding as described in FIG. 11, The number of matrix elements in theset of antenna precoding and distribution matrices or the set ofcombined antenna and tone group precoding and distribution matrices canalso be reduced by reducing the number of tones in each tone group.Moreover, if the matrices for adjacent tone groups are expected to besimilar, the set of matrices may be efficiently compressed by using adifferential encoding technique, in which each matrix is given by itsdifference from the previous matrix. Alternatively, the same matrix maybe used for adjacent tone groups.

While certain preferred embodiments of the invention have beenillustrated and described, it will be clear that the invention is not solimited. Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as defined by theappended claims.

What is claimed is:
 1. A wireless transmitter comprising: tone groupprecoding matrix blocks having inputs coupled to outputs of bitallocation blocks and having outputs, the tone group precoding matrixblocks distributing signal energy across the tone groups for each of themultiple data streams to distribute the data bits across multiple tones;antenna precoding matrix blocks having inputs coupled to the outputs ofthe tone group precoding matrix blocks and having outputs, the antennaprecoding matrix blocks being applied to the tones; and inverse fastFourier Transform and parallel to serial blocks having inputs coupled tothe outputs of the antenna precoding matrix blocks.
 2. The wirelesstransmitter of claim 1 in which there is a separate antenna precodingmatrix block for each tone.
 3. The wireless transmitter of claim 1 inwhich there is one antenna precoding matrix block for a group of tones.4. The wireless transmitter of claim 1 in which 256 available tones aredivided into 32 groups of 8 tones each and there are 32 antennaprecoding matrix blocks, one for each group.
 5. The wireless transmitterof claim 1 in which the tone group precoding matrix blocks distributesignal energy equally across the tone groups for each of the multipledata streams.
 6. The wireless transmitter of claim 1 in which theantenna precoding matrix blocks are applied to the tones to distributethe energy of the tones equally across the outputs of the antennaprecoding matrix blocks.
 7. The wireless transmitter of claim 1 in whichthe tone group precoding matrix blocks, and antenna precoding matrixblocks are free of feedback from a receiver.
 8. A wireless transmittercomprising: tone group precoding matrix blocks having inputs coupled tooutputs of bit allocation blocks and having outputs, the tone groupprecoding matrix blocks distributing signal energy across the tonegroups for each of the multiple data streams to distribute the data bitsacross multiple tones; an antenna precoding matrix block having inputscoupled to the outputs of the tone group precoding matrix blocks andhaving outputs, the antenna precoding matrix block being applied to thetones; and inverse fast Fourier Transform and parallel to serial blockshaving inputs coupled to the outputs of the antenna precoding matrixblock.
 9. The wireless transmitter of claim 8 in which 256 availabletones are divided into 32 groups of 8 tones each and there are 32antenna precoding matrix blocks, one for each group.
 10. The wirelesstransmitter of claim 8 in which the tone group precoding matrix blocksdistribute signal energy equally across the tone groups for each of themultiple data streams.
 11. The wireless transmitter of claim 8 in whichthe antenna precoding matrix blocks are applied to the tones todistribute the energy of the tones equally across the outputs of theantenna precoding matrix blocks.
 12. The wireless transmitter of claim 8in which the tone group precoding matrix blocks, and antenna precodingmatrix blocks are free of feedback from a receiver.
 13. The method ofclaim 8 in which there is a separate antenna preceding matrix block foreach tone.
 14. A method, comprising: receiving at least one data stream;apportioning data bits from the at least one data stream among availabletones; outputting a quadrature amplitude modulation (QAM) constellationsignal point for each tone; distributing signal energy across the tonegroups for the at least one data stream to distribute the data bitsacross multiple tones; and distributing the data bits across multipleantennas.
 15. The method of claim 14 in which there is one antennaprecoding matrix block for a group of tones.
 16. The method of claim 14in which 256 available tones are divided into 32 groups of 8 tones eachand there are 32 antenna precoding matrix blocks, one for each group.17. The method of claim 14 in which the tone group precoding matrixblocks distribute signal energy equally across the tone groups for eachof the multiple data streams.
 18. The wireless transmitter of claim 14in which the antenna precoding matrix blocks are applied to the tones todistribute the energy of the tones equally across the outputs of theantenna precoding matrix blocks.
 19. The method of claim 14 in which thetone group precoding matrix blocks, and antenna precoding matrix blocksare free of feedback from a receiver.